The present invention relates to a cushioning sheet for use in the casting of a denture or the like. The material serves to absorb the difference in the thermal expansion between a casting frame and a casting material.
FIG. 6 shows, as an example, how a denture is cast in the prior art. A casting mold b in which is embedded a denture model a made of wax (including a denture core) is heated so that the model a will be burnt and flown away, and a molten metal is cast in the space formed by the removal of the model a to form a denture.
If the casting material for forming the casting mold b is not strong enough, it is necessary to fill the casting material into a cylindrical casting frame c as shown in FIGS. 5 and 6 to form a casting mold b. However, the use of the casting frame c poses other problems that the casting mold b might be broken or cracked due to the difference in the thermal expansion between the casting frame c and the casting mold b and that the casting material might partially stick out into the space formed by removing the wax model a. In the former case, the mold will become useless. In the latter case, the resulting denture would not comply with the model in shape. Since a denture is intended to be set in the mouth and brought into occlusion with other teeth, high precision is required.
Therefore, the casting frame c customarily has the whole surface of its inner periphery covered with a cushioning material d made of asbestos to absorb the difference in the thermal expansion. However, since asbestos is so harmful to a human body as to deteriorate the health of persons who inhale its powder, its production cost is soaring.
The dental technicians who handle asbestos are exposed to the danger of inhaling not a small amount of asbestos powder. Thus it has been hoped to develop a cushioning material which substitutes for an asbestos cushion.